Bipolar transistors, conventionally bipolar junction transistors (BJTs) but also including insulated gate bipolar transistors (IGBTs), have been used in switched mode power converters (SMPCs) for many years. BJTs in particular are relatively inexpensive and rugged devices, with low on-state resistance and high power density.
The use of a low cost bipolar junction transistor (BJT) as the primary switch in an offline power converter is desirable, since it provides both high breakdown voltage and low on-state voltage. However, difficulties are presented by the BJT's current-controlled nature. Switching the BJT efficiently at the required high frequencies (50-100 kHz) demands careful management of charge into and out of the base terminal. This is of particular concern for modern, low-cost, offline power converters: efficiency requirements, both in active and no-load conditions, along with ever-shrinking form factors demanded by consumer electronics applications, place stringent limits on what is achievable.
In more detail, the bipolar nature of a transistor can lead to high switching (turn-on and turn-off) losses, as during the transitions significant current may flow through the switch whilst there is a voltage across it. Furthermore optimal drive of BJTs is not always straightforward, at least in part due to charge storage in the device and the relatively high power dissipation of their current drive requirements. Charge storage in the BJT may delay the instant at which the switch turns off. This may be achieved internally or intrinsically in the BJT (e.g. in the base region).
On the other hand, switching MOSFETs may have lower switching losses, allowing MOSFET-based SMPCs to run at higher switching frequencies with acceptable efficiency. A higher switching frequency may allow the size of some of the SMPC's inductive components to be reduced. Furthermore, relatively simple, efficient drive schemes are afforded by the MOSFET's voltage-driven nature. For at least these reasons, MOSFETs have replaced BJTs in some SMPC applications, despite the relatively higher cost of a MOSFET.
Attempts have been made to improve BJT drive, both to reduce switching losses and to minimise the power dissipation of the drive circuits. The ‘proportional base drive’ approach makes the base drive current proportional to the load, or collector, current. However a conservative estimate of BJT gain is required in such schemes, to ensure that sufficient base drive current is delivered for all operating conditions and all BJTs within the chosen specification. This inevitably leads to unnecessarily high power dissipation for the majority of operating conditions and BJTs. The consequences of insufficient BJT base drive can be severe, as the switch may become increasingly resistive whilst conducting considerable current. Dramatic switch failure may result.
In view of the above, the fields of transistor control and SMPCs continues to provide a need for an improved transistor drive scheme. A preferred drive scheme may be optimised for, e.g., efficiency, for example of BJT driving and preferably for a wide range of BJTs, and/or for reliability, accuracy, cost, size and/or complexity, etc.
For use in understanding the present invention, the following disclosures are referred to:                U.S. Pat. No. 4,318,011 of Licentia;        U.S. Pat. No. 6,348,819 of Philips;        U.S. Pat. No. 5,017,802 of Siemens;        U.S. Pat. No. 6,377,087 of Philips;        WO83/00590 of Gould Inc;        U.S. Pat. No. 7,218,164 of STMicroelectronics;        U.S. Ser. No. 11/445,473, U.S. Ser. No. 12/405,618 and U.S. Ser. No. 12/752,611, international applications PCT/GB2008/050300 (WO2008/132508 and WO2008/132509), and unpublished provisional application U.S. 61/767,023 filed on Feb. 20, 2013, of Cambridge Semiconductor Ltd.        